1. Field of the Invention
The present invention relates to a liquid crystal display device and, more particularly, to an in-plane switching mode liquid crystal display device capable of maintaining maximum light transmittance.
2. Description of the Related Art
A liquid crystal display device is typically used as a high image quality and low-power flat panel display device. The liquid crystal display device is formed by attaching a thin film transistor array substrate and a color filter substrate facing each other with a uniform interval therebetween. A liquid crystal layer is formed between the thin film transistor array substrate and the color filter substrate. Pixels are arranged on the thin film transistor array substrate in a matrix form, and a thin film transistor, a pixel electrode and a capacitor are formed at a unit pixel. A common electrode for applying an electric field to the liquid crystal layer with the pixel electrode, an RGB color filter for implementing color and a black matrix are formed on the color filter substrate. Alignment films are formed at facing surfaces of the thin film transistor array substrate and the color filter substrate and rubbed to make the liquid crystal layer be arranged in a certain direction.
When an electric field is applied between the pixel electrode formed at each unit pixel of the thin film transistor array substrate and the common electrodes formed at the entire surface of color filter substrate, the liquid crystal is rotated due to dielectric anisotropy, whereby the light is transmitted or blocked by unit pixels to display a character or an image. However, such twisted nematic mode liquid crystal display device disadvantageously has a narrow viewing angle. Thus, recently, an in-plane switching mode LCD which solves the problem of the narrow viewing angle by aligning liquid crystal molecules in an almost horizontal direction with respect to the substrate has been actively studied.
FIGS. 1A and 1B illustrate a unit pixel of the related art in-plane switching mode LCD device. Specifically, FIG. 1A is a plan view and FIG. 1B is a cross-sectional view taken along line I-I of FIG. 1A. As shown in FIGS. 1A and 1B, a gate line 1 and a data line 3 are arranged horizontally and vertically on the first transparent substrate 10, defining a pixel region. In an actual liquid crystal display device, the ‘N’ number of gate lines 1 and the ‘M’ number of the data line 3 cross each other to form N×M number of pixels, but in the drawings, only one pixel is shown.
A thin film transistor 9 consisting of a gate electrode 1a, a semiconductor layer 5 and source/drain electrodes 2a and 2b is disposed at a crossing of the gate line 1 and the data line 3. The gate electrode 1a and the source/drain electrodes 2a and 2b are connected to the gate line 1 and the data line 3, separately. A gate insulation film 8 is deposited over on the entire substrate.
A common line 4 is arranged parallel to the gate line 1 in the pixel region, and a pair of electrodes, namely a common electrode 6 and a pixel electrode 7, for switching liquid crystal molecules are arranged parallel to the data line 3. The common electrode 6 is simultaneously formed together with the gate line 1 and connected to the common line 4, and the pixel electrode 7 is simultaneously formed together with the source/drain electrodes 2a, 2b and connected to the drain electrode 2b of the thin film transistor 9.
A passivation film 11 is formed over the entire surface of the substrate including the source electrode 2a and the drain electrode 2b. A pixel electrode line 14 is formed overlapping the common line 4 and connected to the pixel electrode 7. The pixel electrode line 14 forms a storage capacitor (Cst) together with the common line 4 with an gate insulation film 8 interposed therebetween.
A black matrix 21 for preventing a leakage of light to the thin film transistor 9, the gate line 1 and the data line 3, and a color filter 23 for implementing colors are formed on the second substrate 20, on which an overcoat film 25 for flattening the color filter 23 is formed. Alignment films 12a and 12b for determining an initial alignment direction of liquid crystal are formed at the facing surfaces of the first and second substrates 10 and 20, respectively. A liquid crystal layer 13 for controlling light transmittance by a voltage applied across the common electrode 6 and the pixel electrode 7 is formed between the first substrate 10 and the second substrate 20.
FIGS. 2A and 2B illustrate a driving principle of the related art in-plane switching mode LCD device. FIG. 2A shows driving of the liquid crystal molecules when the voltage is not applied between the common electrode 6 and the pixel electrode 7 and FIG. 2B shows driving of the liquid crystal molecules when a voltage is applied therebetween.
First, with reference to FIG. 2A, when the voltage is not applied to the in-plane switching mode LCD device, the liquid crystal molecule in the liquid crystal layer is aligned along a rubbing direction (the direction of arrow ↑ in FIG. 2A) of the alignment films formed at the facing surfaces of the first and second substrates. With reference to FIG. 2B, when a voltage is applied across the common electrode 6 and the pixel electrode 7, an electric field is generated therebetween and the liquid crystal molecule transmits an amount of light corresponding to the intensity of the electric field.
FIG. 3 is a graph showing light transmittance characteristics according to a drive voltage of an in-plane switching mode LCD device in accordance with the related art. As shown in FIG. 3, as the voltage-applied to the common electrode and the pixel electrode increases, the light transmittance linearly increases. However, if the voltage continues to increase, the light transmittance decreases begins to decrease at 6V. In this case, assuming that the voltage value representing a maximum light transmittance is Vmax, the Vmax voltage value is the voltage at which the liquid crystal molecule makes a 45° with respect to the initial alignment direction of the alignment film.
If a voltage greater than Vmax is applied, the transmittance drops. However, the transmittance obtained in the graph shows an ideal case, and an actual product has maximum luminance at a voltage lower than Vmax, which is a theoretical voltage value. Thus, the in-plane switching mode LCD device has a problem in that the liquid crystal molecules of the liquid crystal layer are always switched on the same plane, reducing the grey level in the vertical and horizontal viewing angle directions and light transmittance at voltages higher voltage than Vmax is less. To solve such problems, a voltage lower than Vmax is set as a Vmax of the actual product, but in this case, there is a high possibility that maximum luminance of the product will not occur. In addition, even though the white state needs to be displayed by applying Vmax, since liquid crystal molecules are collectively arranged in one direction, a screen image viewed in the direction of the shorter side of the liquid crystal molecules has a yellowish and a screen image viewed in the direction of the longer side has a bluish, degrading image quality.